1. Field of the Invention
The present invention relates to a charging device having a charging member held in contact with or in close proximity to an object to be charged so as to charge the same. The present invention also relates to a process cartridge and an image forming apparatus which are equipped with such a charging member.
2. Description of the Related Art
In image forming apparatus, such as electrophotographic apparatus (including copying machines and laser beam printers) or electrostatic recording apparatus, a corona discharge device, which effects corona discharge by applying high voltage to a wire, has widely been used as the means for performing the charging process (which also includes charge removal) on the object to be charged, which object consists, for example, of an image carrying member, such as a photosensitive or dielectric member. The corona discharge device adopts a non-contact type charging system in which the object surface to be charged is exposed to the corona generated by the corona charger so as to be charged thereby.
Currently, the use of contact-type charging means (for contact charging) is increasing. In contact charging, a voltage is applied to a charging member (a conductive member) of a roller-type, blade-type, etc., which is held in contact with or in close proximity to the surface of the object to be charged so as to charge the same.
It is not absolutely necessary for the charging member to be in contact with the surface of the object to be charged. A non-contact state in which the charging member and the object to be charged are in close proximity to each other will suffice as long as the requisite discharge region, which is determined by an inter-gap voltage and a correction Paschen curve, is reliably ensured between the charging member and the surface of the object to be charged. This kind of non-contact charging will also be included in the category of "contact charging" described below.
A charging device of the contact type has the following advantages over the corona discharge device, which is of the non-contact type: the requisite application voltage for obtaining a desired electric potential on the surface of the object to be charged is relatively small; the amount of ozone generated during the charging process is so small as to eliminate the need for an ozone removal filter, thereby simplifying the structure of the gas discharge system of the device; it is maintenance-free; it has a simple structure; etc.
In view of this, contact-type charging devices are attracting attention as a substitute for the corona discharge device to be used as the means for performing charging process on the object to be charged, which consists of a photosensitive member or the like, in image forming apparatus like electrophotographic apparatus or electrostatic recording apparatus, and have actually been put into practical use as such charging means.
In contact charging, the voltage to be applied to the charging member may be a DC voltage (a DC application system) or an oscillating voltage, which is a voltage whose value periodically fluctuates with time (an AC application system).
Regarding the AC application system, the present applicant has made a proposal in Japanese Patent Laid-Open No. 63-149669, etc., according to which charging is executed by applying an oscillating voltage and, in particular, an oscillating voltage exhibiting an inter-peak voltage that is not smaller than double the charging-start voltage, which is a DC voltage applied to the object to be charged to start the charging thereof. This system has proved effective, for it is capable of performing a uniform charging process (including charge removal).
The oscillating voltage is a voltage comprising an oscillating voltage component (hereinafter referred to as the "AC component"), or a combination of such an AC component and a DC voltage component (a voltage corresponding to the target charging potential, hereinafter referred to as the "DC component") superimposed one upon the other. Appropriate examples of the AC component waveform include a sinusoidal wave, a rectangular wave and a triangular wave. A rectangular-wave voltage formed by periodically turning ON/OFF a DC power source will also serve the purpose.
FIG. 11 is a schematic diagram showing an example of the construction of an image forming apparatus employing a contact-type charging device of the AC application system as the charging means. The image forming apparatus of this example consists of a laser beam printer utilizing the electrophotographic process.
A photosensitive drum 100 has a photosensitive layer 101 and a base 102, and rotates in the direction indicated by an arrow A. A charging roller 200, which serves as the charging member, has a core 201 and a conductive rubber 202 and is pressed against the drum 100 by a spring 3. AC and DC voltages are applied to the charging roller 200 from a power source 4. The charging surface of the charging roller 200 is on the opposite side of the drum with respect to a plane which contains a tangent H passing through a point .smallcircle. on the surface of the drum 100 where it is in contact with the charging roller 200. The drum 100, charged by the charging roller 200, is subjected to image exposure that is effected by a laser beam, whereby an electrostatic latent image is formed on the drum. Then, a toner image is formed on the drum by a development sleeve 6. The toner image on the drum 100 is transferred onto a recording paper 7 by a transfer roller 8. After the transfer, the toner remaining on the drum 100 is removed therefrom by a cleaner 9.
The image forming apparatus described above, which uses a contact-type charging device as the means for charging the object to be charged (the image carrying member), has the following problems:
To obtain a stable surface potential, an oscillating voltage is used in the AC application system as the voltage to be applied to the charging member 200. In the surface potential thus obtained, positive and negative voltage components alternately repeat themselves to be concentrated into a DC voltage Vdc, resulting in fine periodic fluctuations appearing in the surface potential.
FIG. 12 is a graph showing such fluctuations in surface potential. In the diagram, the horizontal axis indicates the displacement of the surface of the photosensitive drum 100, which serves as the object to be charged. Here, the displacement of the drum surface, which occurs with the rotation of the drum, is recorded starting from the point .smallcircle., at which the drum 100 is in contact with the charging roller 200. The vertical axis of the diagram indicates the surface potential. In FIG. 12, an area indicated by symbol B represents a charging region, corresponding to the region B in FIG. 11, where charging is actually performed. The potential difference in the fluctuations ranges from several tens of V to one hundred and several tens of V, and the period of the potential fluctuations depends upon the frequency f of the power source 4 and the process speed.
FIG. 13 is a diagram representing the surface of the recording paper 7, on which the above fluctuations in potential are schematically visualized, as indicated at 7a. Assuming that a special pattern having a specific period in the direction in which the recording paper is fed, e.g., a lateral-striped pattern as indicated at 7b, is output onto the recording paper 7, an interference pattern 7c will be generated in the image if the interval of the stripes is close to that of the potential fluctuations on the drum surface.
Due to the restrictions in the precision of the parts, a variation of not smaller than 10% from a predetermined value is inevitable in the AC component frequency of the power source 4. Thus, depending upon the power source, the frequency may be close to the spatial frequency of the stripes, resulting in the generation of a serious interference pattern.
To cope with the problem of such an interference pattern, the present applicant has proposed a system according to which the AC component frequency of the power source to be applied to the charging member is increased in accordance with the process speed. However, the current high process speed, which is a result of the recent increase in the speed of image processing apparatus, has led to another problem that the so-called "charging noise", which is due to a primary power source frequency, increases as this primary frequency increases.